Cryogenic fuse

ABSTRACT

A fuse element is manufactured of lead or other suitable material displaying superconductive properties at very low temperatures. A fuse including such an element is employed in a high voltage transmission line by serially inserting the fuse element in the line and maintaining the element in its superconductive state by using a liquified gas to cool the element during normal current levels. A sensor is provided which develops a control signal in the event excessive current levels are reached. The control signal is applied to control means which automatically vents the fuse by allowing the coolant to escape and thereby raises the temperature of the fuse such that its normal resistivity returns. The resulting I2R heat developed within the element due to the current flow therein will cause the element to immediately vaporize thereby opening the circuit. In other embodiments the control signal can be employed to provide power to a heating element mounted in thermal proximity with the fuse element to increase its temperature in the event of excessive current. Also the control signal can be employed to cause a magnetic field to be developed which causes the fuse to return to its normal resistivity. The fuse and its related circuitry can be designed to provide either a fast-blow or a slow-blow operating fuse.

United States Patent [451 Aug. 15, 1972 Keeler, II

[54] CRYOGENIC FUSE [72] Inventor: Miner S. Keeler, II, 2525 IndianTrail SE, Grand Rapids, Mich. 49506 [22] Filed: Aug. 18, 1971 [21]Appl.No.: 172,714

[52] U.S.Cl ..3l7/13 D, 307/245, 307/306, 317/40A [51] 1nt.Cl. ..1l02h3/08 [58] Field of Search....3l7/13 D, 40 A; 307/245, 306

[56] References Cited UNITED STATES PATENTS 3,202,836 8/1965 Nyberg..307/24SX 3,384,762 5/1968 Mawardi ..307/245 3,579,035 5/1971 Bumier..317/l3D 3,581,113 5/1971 Kafka ..317/l3D Primary Examiner-James D.Trammell Attorney-Price, Heneveld, Huizenga & Cooper [57] ABSTRACT blematerial displaying superconductive properties at very low temperatures.A fuse including such an element is employed in a high voltagetransmission line by serially inserting the fuse element in the line andmaintaining the element in its superconductive state by using aliquified gas to cool the element during normal current levels. A sensoris provided which develops a control signal in the event excessivecurrent levels are reached. The control signal is applied to controlmeans which automatically vents the fuse by allowing the coolant toescape and thereby raises the temperature of the fuse such that itsnormal resistivity returns. The resulting 1 R heat developed within theelement due to the current flow therein will cause the element toimmediately vaporize thereby opening the circuit. In other embodimentsthe control signal can be employed to provide power to a heating elementmounted in thermal proximity with the fuse element to increase itstemperature in the event of excessive current. Also the control signalcan be employed to cause a magnetic field to be developed which causesthe fuse to return to its normal resistivity. The fuse and its relatedcircuitry can be designed to provide either a fast-blow or a slow-blowoperating fuse.

A fuse element is manufactured of lead or other suita- 19 Claims, 4Drawing Figures courkoz. 5O

MEANS USE f Z 51.5mm

g a E CRl'OGEh'IG so /ace so CRYOGENIC FUSE BACKGROUND OF THE INVENTIONThe present invention relates to fuses and particularly to a fusesuitable for use in a high voltage transmission line which employs acryogenic source to maintain a normally resistive element in asuperconductive state during normal current levels.

Present commercially available circuit breakers fo use with high voltagetransmission lines are extremely costly due to the complexity of theirmanufacture which is necessitated by the requirements of reliability ofoperation under various environmental conditions as well as operatingconditions. During electrical storms when voltage or current surges on atransmission line can become rather severe, the circuit breakers have atendency to interrupt momentarily thereby producing high transientvoltages and currents along the transmission line which in someinstances can produce considerable damage to transformers or the like.It is desirable, therefore, to provide a relatively inexpensive circuitinterrupting means which is positive and fast acting in the event of anexcessive current surge thereby eliminating additional current andvoltage pulses caused by the presently available circuit breakersthemselves. A conventional fuse cannot be employed since the magnitudeof current necessary to be carried during the normal operation wouldnecessitate a relatively large fuse element which would not vaporizeimmediately and thereby interrupt the circuit under overload currentconditions. The fuse of the present invention, however, employs thesuperconductive properties of a material such that the fuse element madeof this material will have extremely low conductivity when held in itssuperconductive state by a cryogenic liquid and which can be rapidlyreturned to its normal conductivity state in response to excessivecurrent or voltage surges along the transmission line. When inserted inseries with the transmission line therefore, the fuse element of thepresent invention will provide a conduction path which does notinterfere with the normal operation of the line. In the event ofexcessive current flow, however, the fuse element is returned to itsnormal conductivity state and provides a relatively high resistance. Theresulting I R heat rise in the fuse element due to the currenttherethrough will be sufficient to immediately vaporize the elementthereby opening the circuit.

SUMMARY OF THE INVENTION A fuse embodying the present inventioncomprises a material displaying superconductive properties at extremelylow temperatures and relatively high resistance properties attemperatures above its superconductive temperature region. A source ofcryogenic liquid is provided to maintain the fuse element in itssuperconductive state when pennissible current levels are flowingtherethrough. Additional means are provided for sensing excessivecurrent levels and for returning the fuse element to its normalresistivity state such that it will vaporize due to the heat developedwithin he device by the current flowing therethrough.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates in block diagramform a fuse including a fuse element inserted in series with atransmission line and associated sensing and control means, and thecryogenic source used therewith;

FIG. 2 is a partial cutaway view of one embodiment of the presentinvention;

FIG. 3 is a cutaway end view of another embodiment of the presentinvention; and

FIG. 4 is a diagram in block form showing the use of the fuse of thepresent invention in conjunction with a conventional circuit breaker andthe circuits for providing a control signal which will cause the fuse ofthe present invention to open in the event the circuit breaker has beeninterrupted in a predetermined manner.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION Aconductor 10 of a transmission line is shown in FIG. 1. Inserted inseries therewith is a fuse element 20 which can be manufactured of leadwhich displays superconductive properties at or below 7.2 K. A cryogenicsource 30 is associated with the fuse element and supplies a liquifiedgas such as helium which has a temperature of 4.2 K. when in its liquidstate and which will therefore, when adjacent the fuse element 20, holdthe lead element in its superconductive state. Additionally shown inseries with the transmission line 10 is a sensor 40 for sensing thelevel of current in the conductor 10. Although shown in series with theconductor, it is understood that the sensor need not be electricallycoupled in series and may sense the current in the conductor 10 byinductive coupling to the magnetic field around the conductor.

The sensor 40 produces an output signal which is applied to a controlmeans 50. The control means can operate in a variety of manners torespond to a signal from sensor 40 which is representative of anexcessive current level in a conductor 10 to cause the normalresistivity of fuse element 20 to return. As this occurs, the element 20is instantly vaporized due to the PR heat developed within the elementdue to the resistance (R) of the element. Two ways in which the controlmeans can operate to provide this function are shown in FIGS. 2 and 3,respectively.

FIG. 2 shows a partial cutaway view of the fuse element 20 which issurrounded by an electrical insulator 21 and a heating element 22. Theannular space 24 adjacent the heating element 22 is filled with theliquified helium and holds the fuse element 20 within thesuperconductive temperature ranges of lead from which it is made. Athermal and electrical insulating jacket 26 forms the outer surface ofthe structure. The heating element 22 adjacent the fuse element 20 canbe of the woven conductor type which surrounds the fuse element 20 andwhich is responsive to current from the control means 50 to providesufficient heat to raise the temperature of the lead fuse element 20above 7.2 K.

In this embodiment (FIG. 2) the control means 50 includes a power sourcesuch as a battery pack capable of providing sufficient current to theheating element 22 and which is controlled by the signal from the sensor40 such that current is applied to the heating element 22 in the eventof a current overload in conductor 10. When a current overload occurs,the lead fuse element 20 will be heated above its superconductive regionand become relatively resistive. The transmission line current inconductor which also flows through the fuse element 20 will thereforecause a power or PR loss within the fuse element 20 which will generateconsiderable heat and which will instantaneously vaporize the lowmelting point fuse element 20, thereby opening the electrical circuit inwhich the fuse element is placed. It is noted that the electricalinsulating layer 21 between the fuse element 20 and heater element 22must be sufficiently thermally conductive such that it will notinterfere with the operation of the fuse.

The length of the thermal jacket and fuse element is dictated by thevoltages present along the transmission line. It is necessary that whenthe fuse element 20 melts, a sufficient gap is formed to preventbreakover arcing which would ionize the surrounding atmosphere andprovide a current path across the open fuse. In this regard, theionization potential of helium is considerably greater than that ofnitrogen, the main constituent of air. Thus, by employing helium or thecryogenic medium, the breakover arcing problem due to ionization isconsiderably reduced.

The control means 50 may include wave shaping and clipping circuitry tovary the shape of the signal from sensor 40. Circuit 50 also willinclude means for selectively operating the power source employed toactivate the heater element 22. For example, a relay may be employedwhich couples the current from the power supply in the control means 50to the heating element Instead of employing a heater element 22, aninductor wound around the fuse element 20 could be employed to provide amagnetic field which would return the element to its normal resistivityin the event of an overcurrent in conductor 10. This phenomenon is wellknown and will not be described in detail. It is necessary, however,that the magnetic field produced in the induction in response to currentsupplied thereto from the control means is sufficient to return the fuseelement to its normal resistivity.

FIG. 3 illustrates an alternative embodiment of the present invention inwhich the cryogenic fluid itself is vented from the space between theconductor and the insulating jacket to cause a heat rise in the fuseelement instead of applying heat or a magnetic field directly as shownin the embodiment of FIG. 2. In FIG. 3 the lead fuse element 20 isappropriately suspended within an annular shell 26' which is a thermalinsulator comprising two halves 31 and 33 which are hingedly mounted byhinge means 32 shown at the bottom of the figure. The enclosure 26 issecured by controllable securing means 28 shown at the top of thefigure. Cryogenic fluid in the annular space 24' surrounds the lead fuseelement 20 and maintains the fuse element in its superconductive stateduring normal current levels in the conductor 10 (FIG. 1). In the event,however, that excessive current levels are detected by the sensor 40,the control means can develop a signal which is applied to the securingmeans 28 to thereby explosively or otherwise open the hinged portions 31and 33 of the insulating shell 26 and vent to the atmosphere the highpressure cryogenic gas within space 24'. The securing means 28 maycomprise, for example, explosive bolts or the like which respond to anelectrical impulse applied thereto by the control means 50 toinstantaneously open the halves of the insulating shell. When so openedthe temperature of the lead fuse element 20 will rapidly increasethereby rendering the element relatively resistive and as before, itwill rapidly vaporize due to the PR heat generated therein.

In place of the securing means 28, an electrically operated valve couldbe installed on the jacket 26 such that in the event of an over currentthe valve would open to vent the cryogenic material. In such anarrangement, the hinge 32 would not be necessary.

The embodiments shown in FIGS. 2 and 3 can be mounted in series with theconductor 10 of the transmission line by any suitable meansconventionally known in the art. The cryogenic source 30 may in someapplications includes suitable pump means for circulating liquifiedhelium or the like around and through the fuse. Likewise, the cryogenicsource may include a reservoir of such liquified material and thestructure shown in FIGS. 2 and 3 may include appropriate means forventing gas vapors as the cryogenic liquid tends to boil off. Thefabrication of such equipment is generally well known in the art.

FIG. 4 shows a fuse element and control circuits for providing aslow-blow mode of operation for the fuse. This arrangement may be usedin conjunction with a conventional circuit breaker 55 as shown in thefigure. In this arrangement an electrical connection is made between thecircuit breaker 55 and a gated counter 60. In the event that momentarycurrent surges occur which are not necessarily harmful but which tend tocause the circuit breaker 55 to chatter or otherwise open momentarily,the resultant current interruptions are detected by the circuit. Pulseforming means 57 coupled to breaker 55 provide pulses whose frequencyand duration are directly related to the frequency and duration of thecircuit breaker 55 interruptions. These pulses are applied to a controlelement on the gated counter 60 which is also supplied with clock pulsesfrom an oscillator 58. Thus in the presence of the applied pulses fromthe pulse forming circuit 57 the counter 60 will count clock pulses fromthe oscillator. The resulting accumulated count represents the totalinterrupted time of circuit breaker 5S and is applied to a comparator62.

The comparator is designed such that when the eumulative count from thegated counter 60 reaches a predetermined level the comparator develops acontrol signal which is applied to control means 50. Thus, when it isdesired to cause the fuse element 20 to blow (i.e. open circuit) onlyafter a predetermined number of interruptions of circuit breaker 55 orafter a predetermined accumulated interruption time of the circuitbreaker 55, the output pulse from comparator 62 is employed to cause thecontrol means 50 to open circuit the fuse element 20 in the same mannerdescribed in conjunction with the apparatus shown and described inconjunction with FIGS. 1 through 3.

The control means shown in FIG. 4 additionally may include means forresetting the gated counter 60 periodically by coupling reset pulses tothe counter via line 55. In the event that several momentaryinterruptions of the circuit breaker 55 occur which then cease,

the .gated counter 60 can be reset such that the accumulated count overa predetermined period of time will not trigger the comparator 62 andblow the fuse. A suitable reset pulse generating circuit is includedwithin the control means 50 to provide the desired periodic resetting ofcounter 60. Thus, only current surges on the transmission line which areof a magnitude and frequency against which protective action is desiredwill cause the fuse element to open.

It is noted that the circuitry shown in FIG. 4 could likewise be used inthe circuits shown in FIG. 1 whereby the circuit breaker 55 iseliminated from the circuit and replaced by the sensor 40 shown inFIG. 1. Thus it is seen that the cryogenic fuse of the present inventioncan be operated in either a fast-blow or slowblow mode and either singlyor in combination with a conventional circuit breaker.

Also it can be coupled in series with a circuit closing device where thevoltages are sufficiently high such that the closing device fuses in theclosed position due to arcing during closing and, thus, can not bereopened. The cryogenic fuse is then employed as a breaking device toopen the circuit. Thus, it is seen that the fuse offers flexibleoperating characteristics for use in a variety of transmission lineapplications or in other environments.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows.

1. A cryogenic fuse comprising:

a fuse element comprising a material displaying superconductiveproperties at low temperatures, and relatively high resistive propertiesat temperatures above the superconductive temperature region;

a source of cryogenic liquid;

means for supplying said cryogenic liquid in thermal communication withsaid fuse element thereby holding said fuse element in itssuperconductive state; and

means for returning said fuse element to the normal resistivity state inresponse to predetermined current levels such that said fuse elementwill vaporize due to heat produced in said fuse element by currentflowing therethrough.

2. A fuse as defined in claim 1 wherein said returning means comprisesmeans for venting said cryogenic liquid thereby allowing the temperatureof said fuse element to rise above the superconductive temperatureregion.

3. A fuse as defined in claim 1 wherein said returning means comprisesmeans for applying a magnetic field to said fuse element such that saidelement will return to the normal resistivity state.

4. A fuse as defined in claim 1 wherein said returning means comprisesmeans for increasing the temperature of said fuse element comprisingsensing means for detecting the level of current in said fuse elementand for providing signals representative thereof,

control means coupled to said sensing means and responsive to signalstherefrom such that when excessive current levels in said fuse elementare detected, said control means operates to raise the temperature ofsaid fuse element above its superconductive temperature region.

5. A fuse as defined in claim 4 wherein said control means includes apower source selectively operated to apply power to a heating elementplaced in thermal proximity with said fuse element such that when saidpower supply is activated, said heating element will raise thetemperature of said fuse element.

6. A fuse as defined in claim 5 wherein said fuse element comprises leadand wherein said cryogenic liquid comprises liquified helium.

7. A fuse as defined in claim 4 wherein said control means comprises aninsulating sleeve enclosing said fuse element and cryogenic liquid, saidsleeve comprising first and second portions hingedly mounted one toanother at a first seam therebetween, and bonded to one another bysecuring means at a second seam thereof, said securing means beingresponsive to said signals indicating an excessive current level toexplosively open said insulating shell thereby venting said fuse elementto raise its temperature.

8. A fuse as defined in claim 7 wherein said fuse element comprises leadand wherein said cryogenic liquid comprises liquified helium.

9. A fuse as defined in claim 4 and including further means forselectively causing said fuse element to vaporize only after apredetermined current overload is detected by said sensing means.

10. A fuse as defined in claim 9 wherein said further means includescircuit means for providing a signal to said control means whichindicate an excessive current only after a predetermined number ofcurrent overload pulses of predetermined duration have occurred.

1 l. A fuse as defined in claim 10 wherein said circuit means includes:

a pulse generator,

a gated counter coupled to said pulse generator and operative to countpulses from said generator only in the presence of a signal from saidsensing means which indicates the presence of current overload pulses,said signal from said sensing means being applied to the control elementon said gated counter, and

comparator means coupled to said gated counter and to said control meansfor providing a signal to said control means when a predetermined numberof said pulses from said pulse generator have accumulated in saidcounter.

12. A fuse as defined in claim 11 and further including means forperiodically resetting said gated counter.

13. A fuse comprising a fuse element made of material displayingsuperconductive electrical properties below a predetermined temperature,said fuse element installed in series with an electrical conductor;

means for maintaining said fuse element below said predeterminedtemperature when electrical current of a normal level flows through saidconductor;

sensing means for detecting an excessive current level in said conductorand for producing a control signal in response thereto; and

control means coupled to said sensing means and responsive to saidcontrol signal therefrom to raise the temperature of said fuse elementabove said predetermined temperature such that said fuse element becomesrelatively resistive and vaporizes due to heat generated therein causedby the flow of current therethrough.

14. A fuse as defined in claim 13 wherein said maintaining meanscomprises: a source of cryogenic liquid, means for supplying liquid fromsaid cryogenic source in thermal proximity with said fuse element, andinsulating means surrounding said fuse element and said cryogenicliquid.

15. A fuse as defined in claim 13 wherein said control means comprisesmeans for explosively removing said insulating means such that saidcryogenic liquid will be vented thereby raising the temperature of saidfuse element such that said material will return to its normalresistivityv 16. A fuse as defined in claim 15 wherein said materialcomprises lead and said cryogenic liquid comprises liquified helium.

17. A fuse as defined in claim 13 wherein said control means includes apower supply selectively operated to apply power to a heating elementmounted in thermal contact with said fuse element.

18. A fuse as defined in claim 17 and further including sensing meansfor detecting excessive current in said fuse element and for providing acontrol signal in response thereto to selectively operate said powersupply and thereby provide power to said heating element.

19. A fuse as defined in claim 5 wherein said insula tor comprises asleeve formed of first and second portions joined at a first seamthereof by hinge means, and said means for explosively removing saidinsulating means comprises securing means joining said first and secondsleeve portions at a second seam, said securing means coupled to saidsensing means and responsive to said control signals developed by saidsensing means during current overloads to explosively separate saidfirst and second sleeve portions in the event a current overload isdetected.

1. A cryogenic fuse comprising: a fuse element comprising a materialdisplaying superconductive properties at low temperatures, andrelatively high resistive properties at temperatures above thesuperconductive temperature region; a source of cryogenic liquid; meansfor supplying said cryogenic liquid in thermal communication with saidfuse element thereby holding said fuse element in its superconductivestate; and means for returning said fuse element to the normalresistivity state in response to predetermined current levels such thatsaid fuse element will vaporize due to heat produced in said fuseelement by current flowing therethrough.
 2. A fuse as defined in claim 1wherein said returning means comprises means for venting said cryogenicliquid thereby allowing the temperature of said fuse element to riseabove the superconductive temperature region.
 3. A fuse as defined inclaim 1 wherein said returning means comprises means for applying amagnetic field to said fuse element such that said element will returnto the normal resistivity state.
 4. A fuse as defined in claim 1 whereinsaid returning means comprises means for increasing the temperature ofsaid fuse element comprising sensing means for detecting the level ofcurrent in said fuse element and for providing signals representativethereof, control means coupled to said sensing means and responsive tosignals therefrom such that when excessive current levels in said fuseelement are detected, said control means operates to raise thetemperature of said fuse element above its superconductive temperatureregion.
 5. A fuse as defined in claim 4 wherein said control meansincludes a power source selectively operated to apply power to a heatingelement placed in thermal proximity with said fuse element such thatwhen said power supply is activated, said heating element will raise thetemperature of said fuse element.
 6. A fuse as defined in claim 5wherein said fuse element comprises lead and wherein said cryogenicliquid comprises liquified helium.
 7. A fuse as defined in claim 4wherein said control means comprises an insulating sleeve enclosing saidfuse element and cryogenic liquid, said sleeve comprising first andsecond portions hingedly mounted one to another at a first seamtherebetween, and bonded to one another by securing means at a secondseam thereof, said securing means being responsive to said signalsindicating an excessive current level to explosively open saidinsulating shell thereby venting said fuse element to raise itstemperature.
 8. A fuse as defined in claim 7 wherein said fuse elementcomprises lead and wherein said cryogenic liquid comprises liquifiedhelium.
 9. A fuse as defined in claim 4 and including further means forselectively causing said fuse element to vaporize only after apredetermined current overload is detected by said sensing means.
 10. Afuse as defined in claim 9 wherein said further means includes circuitmeans for providing a signal to said control means which indicate anexcessive current only after a predetermined number of current overloadpulses of predetermined duration have occurred.
 11. A fuse as defined inclaim 10 wherein said circuit means includes: a pulse generator, a gatedcounter coupled to said pulse generator and operative to count pulsesfrom said generator only in the presence of a signal from said sensingmeans which indicates the presence of current overload pulses, saidsignal from said sensing means being applied to the control element onsaid gated counter, anD comparator means coupled to said gated counterand to said control means for providing a signal to said control meanswhen a predetermined number of said pulses from said pulse generatorhave accumulated in said counter.
 12. A fuse as defined in claim 11 andfurther including means for periodically resetting said gated counter.13. A fuse comprising a fuse element made of material displayingsuperconductive electrical properties below a predetermined temperature,said fuse element installed in series with an electrical conductor;means for maintaining said fuse element below said predeterminedtemperature when electrical current of a normal level flows through saidconductor; sensing means for detecting an excessive current level insaid conductor and for producing a control signal in response thereto;and control means coupled to said sensing means and responsive to saidcontrol signal therefrom to raise the temperature of said fuse elementabove said predetermined temperature such that said fuse element becomesrelatively resistive and vaporizes due to heat generated therein causedby the flow of current therethrough.
 14. A fuse as defined in claim 13wherein said maintaining means comprises: a source of cryogenic liquid,means for supplying liquid from said cryogenic source in thermalproximity with said fuse element, and insulating means surrounding saidfuse element and said cryogenic liquid.
 15. A fuse as defined in claim13 wherein said control means comprises means for explosively removingsaid insulating means such that said cryogenic liquid will be ventedthereby raising the temperature of said fuse element such that saidmaterial will return to its normal resistivity.
 16. A fuse as defined inclaim 15 wherein said material comprises lead and said cryogenic liquidcomprises liquified helium.
 17. A fuse as defined in claim 13 whereinsaid control means includes a power supply selectively operated to applypower to a heating element mounted in thermal contact with said fuseelement.
 18. A fuse as defined in claim 17 and further including sensingmeans for detecting excessive current in said fuse element and forproviding a control signal in response thereto to selectively operatesaid power supply and thereby provide power to said heating element. 19.A fuse as defined in claim 5 wherein said insulator comprises a sleeveformed of first and second portions joined at a first seam thereof byhinge means, and said means for explosively removing said insulatingmeans comprises securing means joining said first and second sleeveportions at a second seam, said securing means coupled to said sensingmeans and responsive to said control signals developed by said sensingmeans during current overloads to explosively separate said first andsecond sleeve portions in the event a current overload is detected.